(1) Field of the Invention
The present invention generally relates to an automatic control of a transmission power in a radio device, such as a satellite communication device. More particularly, the present invention is concerned with an automatic transmission level control device used in a multi-carrier radio communication device.
(2) Description of the Prior Art
Recently, there has been a demand to efficiently use frequencies and transmission power in the field of radio communication devices and satellite communication devices. For the above demand, there has been proposed an SCPC (Single Channel Per Carrier) system having a carrier ON/OFF function, or a multi-carrier TDMA (Time Division Multiple Access) system. These systems use a plurality of carriers, and the number of carriers are dynamically changes on the basis of the number of communications. In these systems, it is desired to provide a transmission power automatic control process which makes it possible to keep the communication channels in the stable state and realize stable communications.
FIG. 1 shows a conventional automatic transmission level control device of a multicarrier communication device. The device shown in FIG. 1 comprises a gain controller 1, a transmitter 2 having a converter and a high power amplifier (HPA), and a hybrid circuit (H) 3 for extracting a part of power of a transmission signal output by the transmitter 2. The gain controller 1, the transmitter 2 and the hybrid circuit 3 form a high-frequency part of the communication device. The device shown in FIG. 1 further comprises a detector (DET) 4 and a control circuit (CNTL) 5. The detector 4 detects the signal from the hybrid circuit 3 and generates a DC voltage based on the detected signal. The control circuit 5 controls the gain of the gain controller 1 on the basis of the DC voltage generated by the detector 4 and information indicative of the number of carriers supplied from a terminal equipment (not shown for the sake of simplicity).
In the multicarrier communication system, the number of carriers is dynamically changed on the basis of the number of communications, as has been described previously. In the conventional multi-carrier communication device, the gain controller 1 is not controlled by only the magnitude of the output signal from the transmitter 2, but taking into account the information indicative of the number of carriers supplied from the terminal equipment. Hence, it becomes possible to generate an output signal of the transmitter 2 based on the number of carriers. As a result, it becomes possible to prevent a change in power of each carrier based on a change in the number of carriers from being recognized to be a simple level variation and keep the transmission level equal to an appropriate level.
As described above, it is necessary to obtain, from the terminal equipment, information indicative of the number of carriers. In order to transfer the above information to the automatic transmission level control device by means of a parallel transmission system, it is necessary to use control lines equal in number to carriers. If a serial transmission system is used to transfer the above information, it is necessary to use a parallel-serial conversion circuit although a single control line can be used. If the information about the number of carriers is transmitted in digital form, it is necessary to simultaneously transfer four bits in a case where nine carriers can be used at maximum.
In practical transmission systems, the high-frequency part including, as a major part, the transmitter, is installed in a place remote from a place where the terminal equipment having modulators and demodulators is installed. For example in a very-small satellite terminal (VSAT) system, the high-frequency part is placed outdoors together with an antenna, and the terminal equipment is placed indoors. In some cases, the distance between the high-frequency part and the terminal equipment is equal to 100 to 200 meters. In the parallel transmission system, it is very expensive to provide a plurality of cables between the high-frequency part and the terminal equipment. The serial transmission system is less expensive than the parallel transmission system because only one cable can be used. However, the serial transmission system needs the parallel-to-serial converter, as has been described previously. Hence, the serial transmission system has a complex hardware structure. In addition, use of the parallel-to-serial converter increases the cost of building up the system. It may be possible to superimpose carriers on signals in the serial transmission system. However, for this purpose, there is a need for a modulator and a hybrid circuit.